practice midterm II solution

practice midterm II solution - ‘ Name a j CHE/ME 109 Exam...

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Unformatted text preview: ‘ Name a j CHE/ME 109 Exam II Section II ' Please CIRCLE your answers. Good luck! . _ . ' I V ' ‘ IV I » I I ProblemI ' - - ’_ 1 _ is - l' I" _ > 5 pts. (a) A hot surface is cooling by natural convection. The surface is 77°C and the air I l of heat transfer? Be sure to igclude the proper units. p 1 g ‘ . L ' fl 1 - ’ I 571%, w/S ‘ v W. iii... 32 18K k= -O’Z:+S L’s/th _ _ V 10 pts. (b) If the surface is vertical and has dimensions of 0.5 m x 0.5 m, determine ' whether the air flow is laminar or turbulent. To receive credit, you need to calculate the appropriate dimensionless number for this problem to justify your answer. * I Chanel—en‘ylx'g 93nd = :5M 3 I l l I a 4— l.C527</0'5 1441/5)?— __.——-———‘——— temperature is 27°C. What are the properties of the air for the determination of the rate ‘ 9 10 pts. (c) If a fan were placed at the base of the surface so that the flow would be up along the surface vertically, what would be the minimum velocity of the fan at which you could neglect the natural convection heattransfer? i VC‘L Ool 'l’O “4+1de ConVefi-fioy‘ Qa ,_.__/<o.| . ‘ ~, 5 Rel U421, 543248340“ Re: v EL/____S_'__,————:V _ J. a MW j Cenozch‘qn _ s. Problem II. Short answer. ' 5 pts. 1. When is heat transfer through afluid conduction and when is it convection? 14‘ m “wick is moving 1+ wlu Imme- , COHVCC‘hbn Wi‘ MIAS‘CCJ: H can '- ' I move, \0 Mofi‘hn of by > bwcjflrkc; curve/n+5. 5 pts. 2. What is the significance of the critical Reynolds Number? I, v m crat‘ml ,Ranchij number V , [(Acitgccnag' 4/141 Mng'Hfi-op a4“ ‘Giow ‘r‘ij'l/VW» - Lou/\Q'V‘CW‘ {*0 hILuU/k't, MA MLCU/ +urloulem+ .v.-._-.._._~__—...__.——v . . v....‘.-r._.. . _ ~‘1J .024 m ' ' Problem In " ' Tu % 27°C Transistor 20 pts. Illa. A commercially available heat sink is used to cool a transistor. If the surface of the heat sink is the same temperature as the transistor(i.e. the fins are 100% efficient), determine the maximum power of the transistor if the surface temperature is not to exceed 77°C. For simplicity, ignore the surface area of the transistor to calculate the area for heat transfer of the base plate. Calculate the amount of heat transferred by natural convection. All of the surfaces are vertical hot plates. m a >96¢9K 124.222: ->‘> taunt an, Pm we» 59 Wag M a. H "ARA AAmk‘u ,oZ4°,o%"U+£d%o‘o;§-] :. , O 101 A," Gr: E1 ‘3 (5 [80‘3[ 0% m]; (a ’- ' S" ' ’ _, I..GIQ7~X/0 L ID . '[fiZZXlo’ w 5 _ law‘n‘t‘" I) N I 5 2,0735 - k; Emu ': é? :hAETS'ToB =64} .ololml goo 9 Eff u) V 10 pts. M b. Calculate the amount of heat transferred from the heat sink by radiation. Consider only the outer-facing sides of the two fins (i.e. use the area for radiation heat a. _ -aX Dowager- QflA:€UA(E my) {smegma _ . fl 3004) 24,11 \N “Mm ._.-—.i .--- ..__< .u.«-w.-£.-aw.fi_p “_-~ ._._._——__._. ._ .._.__.._.—...' _,__.‘ ' '1'! Problem I V. I 15 pts. A SUNNY computer has 1 PCB which generates 80 of power. The air duct has a cross sectional area of 6.3 x 10-5 m2 . The maximum temperature of the room air is 35°C and the maximum allowable exit temperature of the air is 38°C. Assume the air is at 1 atm of pressure. Determine the appropriate volumetric flow rate of the fan to 3 a? + ?5 cool the computer. Tame : :SH‘DK CV: IGOQ d/Eflt saw. 1.143 {a \'/—,‘QY:'_ -_~ files/$1 =‘,(9232,W3— (O l I43 “3 ‘ ' 5 ..' '5» - ' , ' . _.._.._.___.._———.—.._..u—"_. . . .-.. ..._...._.—__...._ .._-. lVb. 15 pts. There is one component in the computer that generates 5 W of power. The component is placed close to the exit of the computer and a heat sink is attached. Ifthe surface of the component is not to exceed 75°C, calculate the minimum surface area of the heat sink. Assume the heat sink is 100% efficient. Use 0.003 m2 as the cross sectional area of the air channel. Ifyou were not able to calculate the volumetric flow rateinparta, use V=2x10~2m3/s, TM cal/mac! . . _ 5w abate-Two T5162 newt R>7rg+3g= 330K . 2’ “Mama FOYCaZi “Whit-C.an ‘ -' V0,: V ‘7 drum/5 7 _ .OOva-z’_ Re ">- \Ia 5 -_.- (433 ES‘X‘O?‘ m) g am I Nu 9,0634 Reqlprl/z "p: E3l2‘/i[308‘(zjcggfl» E" g c 53% W? @122: [elieefiwcgez‘lc] ! ' ' 102M ...
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practice midterm II solution - ‘ Name a j CHE/ME 109 Exam...

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